A number of medically recognized techniques are employed for cataractic lens removal, such as phacoemulsification, mechanical cutting or destruction, laser treatments, water jet treatments, and so on.
The phacoemulsification procedure entails making a corneal incision and inserting a phacoemulsification handpiece into the ocular region, where the handpiece includes a needle that is ultrasonically driven in order to emulsify, or liquefy, the lens. Concomitantly, fluid is irrigated into the eye and the irrigation fluid and liquefied lens material are aspirated from the eye. Other medical techniques for removing cataractous lenses also typically include irrigating the eye and aspirating lens parts and other liquids. Additionally, some procedures may include irrigating the eye and aspirating the irrigating fluid without concomitant destruction, alteration or removal of the lens. As is well known, for these various techniques it is necessary to maintain a stable volume of liquid in the anterior chamber of the eye and this is accomplished by irrigating fluid into the eye at the same rate as aspirating fluid and lens material.
During this procedure, it is possible for the aspirating phacoemulsification handpiece to become occluded. This occlusion is caused by particles blocking a lumen or tube in the aspirating handpiece. Such blockage can result in increased vacuum (i.e. increasingly negative pressure) in the aspiration line. The longer the occlusion is in place, the greater the vacuum. Once the occlusion is cleared, a resulting rush of fluid from the anterior chamber into the aspiration line can outpace the flow of new fluid into the eye from the irrigation source. The resulting imbalance of incoming and outgoing fluid can create a phenomenon known as post-occlusion surge or fluidic surge, in which the structure of the anterior chamber moves rapidly as fluid is replaced. Such post-occlusion surge may lead to eye trauma. Current precautions against post-occlusion surge cause cataract surgery to be lengthier and more difficult for the attending surgeon.
Alternate surgical procedures when an occlusion occurs typically include a reduction of aspiration rate to a level less than the irrigation rate before continuing the procedure. This can be accomplished by changing the aspiration rate setting on the system. This, in turn, allows the pump to run slower and the fluid volume in the anterior chamber to normalize. Other alternate surgical systems may employ a restriction in the aspiration circuit to constrict surge flow when an occlusion clears from the aspiration tube. Alternative techniques heretofore utilized to avert blockage issues of the type described include a reduction of vacuum on the occlusion by adjusting system settings. This technique often requires an assistant to perform the actual modification of settings. Still another technique for vacuum control can be accomplished by reducing pressure on a control footpedal or releasing a footpedal altogether. This technique, however, requires a surgeon to discontinue applying ultrasonic power temporarily until the occlusion is either cleared or has been released from the aspirating phacoemulsification handpiece. A disadvantage in releasing the footpedal is the fact that cataract lens material in the aspirating phacoemulsification handpiece may flow back into the eye chamber.
In addition, a combination of the hereinabove recited techniques may be employed as well. However, once an occlusion occurs, the surgeon must identify the cause and then take corrective action. The length of time before the occlusion clears varies. In the time it takes for a surgeon to identify the cause and request corrective action, the occlusion can build sufficient vacuum and then clear, thus resulting in post occlusion surge. As a result, surgeons tend to operate their phacoemulsification systems at lower vacuum levels than otherwise preferable in order to avoid this problem.
A system and method for improving the phacoemulsification procedure, and specifically the removal of occlusions, is addressed in U.S. patent application Ser. No. 11/086,508, filed Mar. 5, 2005, entitled “Application of vacuum as a method and mechanism for controlling eye chamber stability,” inventors Michael Claus, et al., published as U.S. Patent Publication 20060224143, the entirety of which is incorporated herein by reference. In the '508 application, duration of an occlusion is determined from the sensed vacuum level (typically a rise in vacuum pressure, (i.e. an increasingly negative pressure)) and/or a sensed flow rate (i.e. a drop in flow rate for a constant vacuum pressure), and in response thereto, at least one of the 1) supply of irrigation fluid, 2) vacuum level, 3) aspiration rate, and 4) power applied to the handpiece is/are controlled.
In particular, when an occlusion is encountered and the monitored vacuum level increases, an occlusion threshold value representing the value of the monitored vacuum level at which the aspiration tube has been completely or substantially (e.g., greater than 50%, and preferably greater than 80%) occluded is assessed. If vacuum reaches a maximum allowable vacuum (Max Vac), the pump is typically stopped. A Max Vac setting may be pre-determined or programmed in the system by a user before or during a surgical procedure. The occlusion threshold may be set at or below the same level as the Max Vac setting. In some embodiments the Max Vac level and occlusion threshold value are set to the same level. Alternately, the occlusion threshold value is set at a percentage (i.e. less than or equal to 100%) of the Max Vac level, such as, for example, in a range between about 20% to about 95%. Alternately, the occlusion threshold may be predetermined at or programmed to a set vacuum level.
For systems using vacuum pumps (e.g., Venturi pumps), flow rate is monitored instead of vacuum level. When the aspirating handpiece becomes occluded, i.e. partially or fully blocked, flow rate decreases. An occlusion flow rate threshold value may be pre-set in the system or entered into the system. The occlusion flow rate threshold value is the value at which the flow rate is recognized by the system and/or user as indicating that an occlusion has occurred. In other words, as the monitored flow rate decreases, the occlusion flow rate threshold value is the value of the monitored flow rate at which the aspiration tube has been completely or substantially occluded. In embodiments for combination systems using vacuum pumps and flow pumps, one or both of the vacuum level and flow rate may be monitored and the above-described methods of determining occlusion are employed.
Normal operation of the '508 system or systems like the '508 system is shown in FIG. 1. As time progresses and the occlusion encountered, vacuum pressure rises to a Max Vac level, which is sensed, and the vacuum level backed off with ultrasonic energy applied to break up the occlusion. An occlusion ultrasonic energy mode is initiated when the vacuum falls below the occlusion flow rate threshold or “up” threshold, and vacuum maintained at specified levels in an effort to break up the occlusion. Once the occlusion breaks or is believed broken, the occlusion mode is turned off and pressure falls below the Min Vac or “down” threshold. Unoccluded operation resumes, as the occlusion is believed to have broken.
The problem in some instances is that while the occlusion may break, it does not break significantly with only application of phaco power while vacuum is at the upper threshold and thus the occlusion is not fully engaged with or partially released from the phaco tip. Such an inability to aggressively address and remove the occlusion may interfere with further phaco procedures. In other words, application of energy or phaco/ultrasonic power during the period when pressure is dropping from a Max Vac setting is inadequate, or has been deemed of concern by certain surgeons because it simply results in thrusting the phaco tip into an occlusion that is floating or released from the tip. Further, it is noted that ultrasonic energy application can cause excessive heat application to the region, which is undesirable.
While some surgeons have attempted to address this issue by modulating power using a device such as a footpedal, due to the inherent reaction time of the surgeon when the occlusion is encountered, it is difficult or impossible for an individual employing the system to successfully coordinate vacuum and energy application to effectively address the occlusion.
Thus potential issues with a design such as presented in the '508 application include the fact that an ultrasound energy occlusion mode is activated when reaching an intermediate vacuum level or Mid Vac, also known as CASE Vac, after peak vacuum level is reached. In the FIG. 1 arrangement, the vacuum drops below the low vacuum threshold Min Vac before the system can reset back up to or re-achieve Max Vac. The surgeon and system are restricted to the lower vacuum region, in many cases below the Mid Vac or CASE Vac region, and even after the occlusion has been broken, the surgeon must activate ultrasonic energy to cause the occlusion to leave the tip. The net result of this implementation is that the occlusion is inefficiently addressed, inhibiting successful surgery unless significant energy is applied, such as ultrasonic energy, to the ocular region.
It would be desirable to offer a design wherein the foregoing drawbacks could be addressed and a more effective design provided, such as where a surgeon could fully address the occlusion without being forced to address the occlusion in the low pressure unoccluded mode.